Shore Hardness

I don't believe that the Shore hardness # should enter into your shrinkage calculations at all. There are too many other factors involved when molding the elastomer.

You should select the desired hardness and get information from the supplier of the material regarding mold shrinkage.

1- the table is NOT for elastomer so that do not be surprised. The durometers are different.

2- why do you need the E for EPDM ? do you want to make a FEA simulation ? If it is the case you MUST consider the non linearity and the temperature effect which is VERY important.

3- If you want to use a standard seal look at the application and make the choice according to recommendations.

4- What kind of shrinkage do you think of ? If when for the form then you do not need it, if you want to compute (simulate) compression then you need it.

I want to seal between two platforms. the surface finish is not very straight so I want the EPDM to close all the gaps.

I'll still don't know the dimension if the highest "bump" (I don't know the word for it in English), I just wanna know how do I know how much the EPDM shrink while applying pressure on it according to the shore number (if it possible) and if not, how do i do it.

I do need the E, the choice of material wasn't mine… (Not enough rank yet).

Compressive modulus of elasticity is what you want, not shrinkage.

this was the formula i was planning to use after i get the Young's modulus from the shore number, but how do I get the Young's modulus from the shore number?

Forget the Shore number. It has no bearing on compression. It is a measurement of penetration resistance.

Compression!

Yes, BUT there is a relationship between "intrinsic material stiffness" which measured by the Young modulus and the "penetration" used by the Shore durometer. I shall have a look I had such an indication but I do not know now where it is. I think that with google 's help it could be possible to find even more modern documentation.

Correct. But the OP seems to be focused on Shore hardness when he should be thinking about Young's modulus.

ok.

So I need to calculate how much is the Young's modulus I need from the displacement equation and ask the manufacture to give me the material with this number?

Or Is there any table or any other accessorize that can help me with it?

the equation from wikipedia is the equation?

Yes but it is only an approximation the real value can differ. There is more difficult problem in the computation the fact that elastomer's have a Poisson' s coefficient near to 0.5 (do not change volume under compressive strain) and that the "apparent" module depends on the friction between seal and surfaces. The seal will be in the assembly "stiffer" since it cannot expand due to friction. The magnitude of this "stiffness complement" depends on the seal geometry. This is the reason why in the design of elastomer damper the stiffness is computed considering a form factor.

Normally you can't tell the shrink by the hardness. Shrink is usually associated with the Polymer family and is really dependant on the compound ingredients and the method of processing (ie;plasticisor content, cure cycle and post cure). Think of it like this, the more volitile additives in the compound, the more to outgas during cure, and the greater the shrink. Higher processing temperatures and post curing may add to this effect. Some Silicone compounds may shrink up to 3.2%, while your Nitrile & EPDM compounds typically shrink at APPROXIMATELY 1.8% (thats .018 of an inch per linear inch of seal). Other processing factors include the molding method, ie; compression, which way is the grain of the preform running? The only real way to determin the real shrink factor is to run a study on the compound/process and obtain results by measuring the specimens.

Guest,

I believe that Avisar has confused the terms here. I get the impression that he may be new to elastomers. While he is referring to shrinkage, I believe that he is really concerned about compression in service and is incorrectly using the terms.

Shrinkage, as you have indicated, is more of a processing issue and once the part is molded it is no longer a factor, except in the sense of thermal shrinkage, which can be fairly high with elastomers.

I feel that he is really trying to predict the amount of compression the part will see when put into service.

Indeed i do.

The approximation equation is fine with me (I think) coz i'm planning to use some safety factor for this.

The seal geometry is a square shape and it suppose seal a glass surface and a ULTEM surface.

Or maybe I'm asking the wrong questions and I need to know something or consider something else while approaching this problem or I'm just confused?